Friday, June 13, 2008

Re-opening the Pluto Controversy

A Hubble Space Telescope Map of Pluto Image Credit: Alan Stern, Marc Buie, NASA and ESA

Just when it seemed like the issue of whether Pluto is a planet or not was fading away, it was re-opened by the very body that demoted Pluto in the first place. Earlier this week, the International Astronomical Union (or "IAU" for short), the "ruling body" of astronomy world wide, announced that objects like Pluto shall henceforth be known as "plutoids." This decision was announced by the IAU working group tasked with trying to classify objects, and it was quite unexpected. Most of us didn't even know that such a term was under discussion.

The astronomers who still argue that Pluto is, indeed, a planet came out with some fairly strong condemnation of this announcement. Alan Stern, former head of NASA's science directorate and lead scientist of the New Horizons mission (a robot en route to explore Pluto), said, "It's hard to find anyone who thinks this is (i) necessary, (ii) a step forward, or even (iii) useful." From a scientist, that's pretty strong language. An article published on continues by arguing that a competing body to the IAU is needed.

Frankly, I think those involved need to take a deep breath. I agree that the phrase "plutoid" is stupid; if nothing else, it sounds stupid -- more like a medical condition than a type of heavenly body. (Imagine how you would feel if your doctor came in to a waiting room and said, "I'm sorry, Mrs. Smith, but you have plutoids.") Second, it is actually an over-classification. "Plutoid" is meant to describe icy objects beyond the orbit of Neptune that are large enough that gravity makes them round. But the existing terms "Kuiper Belt Object" already refer to small objects outside the orbit of Neptune, and "dwarf planet" was introduced by the IAU just a year or two ago to describe objects too small to be planets but still large enough that gravity makes them round. I don't know what use a term describing the intersection of these two groups is.

But the other side should calm down, too, I think. The last thing we need is another ruling body to compete with the IAU. The IAU is indeed far from perfect (I think it spends a little too much effort on bureaucracy and not enough on advocacy), but the IAU is not destroying the science of astronomy, and a second major international group would only serve to muddy the waters even more. Let's look at another oft-maligned international body: the United Nations. Most people agree the United Nations is far from perfect, but imagine how much worse the mess would be if we instituted a competing "League of Nations 2". Countries would have to choose between bodies, or maybe belong to both, and then what happens when the two bodies are at cross purposes? Or both trying to work on the same problem but spending more time arguing about who has jurisdiction? That would be worse than a mess; it would be disastrous.

Frankly, the problem is that there is no single good definition for a planet; we humans just have a penchant for trying to classify things that, ultimately, may be unclassifiable. The best classification scheme for planets may be one involving formation scenarios and gravitational interplay and past history of individual objects. But outside of our own Solar System, we have no means of determining most of these things. And even within things we all agree are planets, there are huge differences. Earth is much different from Neptune, which is much different from Jupiter. I'm sure that if Jovians existed, they would be insulted that we would have the gall to compare our puny blue rock to Jupiter's mighty bloatedness.

As humans, we've come to accept that, sometimes, there are not black and white answers, but that most answers come in shades of gray. The same is true even in science. "What is a planet?" Is a question that doesn't have a single answer, and the answer will differ from person to person, and yet all of those answers may be right. Before we go yelling at each other over what Pluto is, we should perhaps ask another question, "Does it matter?" Whether we call Pluto a plutoid, or a Kuiper Belt Object, or a dwarf planet, or a planet, or a "remarkably big chunk of ice and rock really far away" won't change how scientists view and understand the object.

Just don't call Pluto late for dinner.


  1. More accurately, it was Alan Stern who suggested an alternate body to the IAU -- not the article itself. Anyway, I agree with the rest of your post, and made a comment to that effect on the relevant site

  2. The problem with the IAU is the closed, secretive process they continue to use to make these determinations. Even the original vote in Prague only included four percent of the IAU's membership because the organization has no provision for electronic voting. When a professional organization comes up with a sloppy definition that 96 percent of its members had no say in creating, there is clearly a problem.

    What is wrong with keeping the term planet as a broad category encompassing all non-self luminous spheroidal bodies orbiting stars? We could then create multiple subcategories based on issues such as composition and orbital dynamics. For example, there would be terrestrial planets, gas giants, ice giants, and dwarf planets, the latter still being a subclass of planets, but used to describe smaller objects that do not dominate their orbits. We do this type of classification for stars and galaxies, so why not for planets? Why the need for the term planet to be so narrow and exclusionary?

    This issue is not going to fade away because nothing has been resolved. Maybe we should just agree to keep it an open debate until New Horizons gets to Pluto and Dawn gets to Ceres, at which time we will know far more about both bodies than we ever have.

  3. Anonymous4:55 PM

    go get them Korn